Bulk material irradiation system and method

ABSTRACT

A bulk material irradiation system includes an input for inserting bulk material. A bulk material tube is connected to the input, forming a path for bulk material flow. A pressurizing assembly is connected to the bulk material tube for forcing the bulk material to flow through the bulk material tube. An irradiation assembly provides ionizing radiation to irradiate the bulk material passing adjacent to the irradiation assembly in the bulk material tube. Irradiated bulk material exits the bulk material tube through an output.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims the benefit of Provisional Application No.60/184,794 filed Feb. 24, 2000 for “Material Handling System And MethodFor Irradiation” by S. Lyons and S. Koenck, and also claims the benefitof Provisional Application No. 60/192,872 filed Mar. 29, 2000 for“Irradiation Control And Calibration System And Method” by S. Lyons, S.Koenck, B. Dalziel, D. White and J. Kewley, and also claims the benefitof Provisional Application No. 60/208,700 filed Jun. 1, 2000 for “BulkMaterial Irradiation System And Method” by S. Lyons, S. Koenck, B.Dalziel, D. White and J. Kewley, and also claims the benefit ofProvisional Application No. 60/214,697 filed Jun. 27, 2000 for “BulkMaterial Irradiation Exposure Compensation System And Method” by S.Lyons, S. Koenck, B. Dalziel, D. White and J. Kewley, and also claimsthe benefit of Provisional Application No. 60/246,467 filed Nov. 7, 2000for “Bulk Material Irradiation Exposure Compensation System And Method”by S. Lyons, S. Koenck, B. Dalziel, D. White and J. Kewley.

INCORPORATION BY REFERENCE

The aforementioned Provisional Application Nos. 60/184,794, 60/192,872,60/208,700, 60/214,697 and 60/246,467 are hereby incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a bulk material irradiation system andmethod, and more particularly to a system for transporting andirradiating bulk material in a manner such that a precisely controllabledose of irradiation is efficiently delivered to the material.

Irradiation technology for medical and food sterilization has beenscientifically understood for many years dating back to the 1940's. Theincreasing concern for food safety as well as safe, effective medicalsterilization has resulted in growing interest and recently expandedgovernment regulatory approval of irradiation technology for theseapplications. United States Government regulatory agencies have recentlyapproved the use of irradiation processing of red meat in general andground meat in particular. Ground meat such as ground beef is ofparticular concern for risk of food borne illness due to the fact thatcontaminants introduced during processing may be mixed throughout theproduct including the extreme product interior which receives the leastamount of heat during cooking. Irradiation provides a very effectivemeans of reducing the population of such harmful pathogens.

The available sources of ionizing radiation for irradiation processingconsist primarily of gamma sources, high energy electrons and x-rayradiation. The most common gamma source for irradiation purposes isradioactive cobalt 60 which is simple and effective but expensive andhazardous to handle, transport, store and use. For these reasons,electron beam and x-ray generation are becoming the preferredtechnologies for material irradiation. An exemplary maximum electronbeam energy for irradiation purposes is on the order of 10 millionelectron-volts (MeV) which results in effective irradiation withoutcausing surrounding materials to become radioactive. The necessaryelectron beam power must be on the order of 5 to 10 kilowatts or more toeffectively expose materials at rates sufficient for industrialprocessing.

Electron beam and x-ray irradiation systems both employ an electronaccelerator to either emit high velocity electrons directly forirradiation or to cause high velocity electrons to collide with a metalconversion plate which results in the emission of x-rays. A number ofelectron acceleration techniques have been developed over the pastseveral decades including electrostatic acceleration, pumped cylindricalaccelerators and linear accelerators.

Electrostatic accelerators are characterized by the use of a directcurrent static voltage of typically 30 to 90 kilovolts which accelerateselectrons due to charge attraction. Electrostatic accelerators arelimited in maximum energy by the physical ability to generate and managehigh static voltage at high power levels. Electrostatic acceleratorsusing Cockroft-Walton voltage multipliers are capable of energy levelsof up to 1 MeV at high power levels, but the 10 MeV energy levelutilized by many systems for effective irradiation is not typicallyavailable.

Various types of pumped cylindrical electron beam accelerators have beenknown and used for many years. These accelerators generally operate byinjecting electrons into a cylindrical cavity, where they areaccelerated by radio frequency energy pumped into the cylinder. Once theelectrons reach a desired energy level, they are directed out of thecylinder toward a target.

RF linear accelerators have also generally been in use for many yearsand employ a series of cascaded microwave radio frequency tunedcavities. An electron source with direct current electrostaticacceleration injects electrons into the first of the cascaded tunedcavities. A very high energy radio frequency signal driven into thetuned cavities causes the electrons to be pulled into each tuned cavityby electromagnetic field attraction and boosted in velocity toward theexit of each tuned cavity. A series of such cascaded tuned cavitiesresults in successive acceleration of electrons to velocities up to the10 MeV level. The accelerated electrons are passed through a set oflarge electromagnets that shape and direct the beam of electrons towardthe target to be irradiated.

A typical industrial irradiation system employs an electron beamaccelerator of one of the types described, a subsystem to shape anddirect the electron beam toward the target and a conveyor system to movethe material to be irradiated through the beam. The actual beam size andshape may vary, but a typical beam form is an elliptical shape having aheight of approximately 30 millimeters (mm) and a width of approximately45 mm. The beam is magnetically deflected vertically by application ofan appropriate current in the scan deflection electromagnets to causethe beam to traverse a selected vertical region. As material to beirradiated is moved by conveyor through the beam, the entire volume ofproduct is exposed to the beam. The power of the beam, the rate at whichthe beam is scanned and the rate that the conveyor moves the productthrough the beam determines the irradiation dosage. Electron beamirradiation at the 10 MeV energy level is typically effective forprocessing of food materials up to about 3.5 inches in thickness withtwo-sided exposure. Conversion of the electron beam to x-ray irradiationis relatively inefficient but is effective for materials up to 18 inchesor more with two-sided exposure.

The prior art industrial irradiation systems previously described aretypically relatively inflexible and require careful setup, calibrationand operation to deliver the irradiation dosage required for safe,effective sterilization. The output energy levels are established by thestructure of the accelerator and are relatively constant. The outputpower levels are determined by equipment settings and calibration andmay vary significantly.

Prior art irradiation systems of the direct electron beam type typicallyemploy electron beam accelerators to generate a stream of electrons atenergy levels of a maximum of 10 MeV. Scanning of the electron beam isperformed using magnetic deflection similar to the type used fortelevision raster scan. The dosage of irradiation delivered to a productpassing by the accelerator is determined by the power of the beam, thebeam scanning speed and the rate that the product is moved by theconveyor through the beam. This dosage is typically set manually by anoperator for a given material to be irradiated, and is expected toremain constant at that setting. While this type of system can delivereffective radiation for a homogeneous product line, there are a numberof shortcomings associated with the system. First, there are a number offactors that may cause the output power to vary after being set by theoperator, including changes in temperature of critical components orshifting of frequency of the critical radio frequency acceleration drivesubsystem. Second, it is cumbersome and inefficient to change theirradiation dosage to be delivered by the system if some differentproduct is to be irradiated that requires different exposure. Thischaracteristic of prior art systems generally dictates that the productmix to be irradiated can change very little during the course ofprocessing. Third, there is no indication that irradiation exposure hasbeen delivered to the products. Physical dosimeters must be placedperiodically on the conveyor or within packages of products and examinedto determine that products have indeed been irradiated at the specifieddosage. Until the dosimeters have been verified, all product that haspassed through the irradiation system must be held in quarantineawaiting verification that the processing was successful. If there is afailure indicated by an underexposed trailing dosimeter, all of theproduct that is held in quarantine is of unknown status, with someamount at the front of the batch probably exposed and some amount at theback of the batch probably unexposed. Depending on the severity of theunknown product irradiation implications, the entire batch may have tobe destroyed.

A conveyor-based irradiation system that addresses many of theshortcomings of prior art systems is disclosed in U.S. application Ser.No. 09/685,779 filed Oct. 10, 2000 for “Irradiation System And Method”by S. Lyons, S. Koenck, B. Dalziel and J. Kewley, which is herebyincorporated by reference. Improvements in the state of the art may alsobe achieved in a bulk material irradiation system, which is the subjectof the present invention.

BRIEF SUMMARY OF THE INVENTION

The present invention is a bulk material irradiation system. An input isprovided for the insertion of bulk material. A bulk material tube isconnected to the input, forming a path for bulk material flow. Apressurizing assembly is connected to the bulk material tube for forcingthe bulk material to flow through the bulk material tube. An irradiationassembly provides ionizing radiation to irradiate the bulk materialpassing adjacent to the irradiation assembly in the bulk material tube.Irradiated bulk material exits the bulk material tube through an output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a front section view, and

FIG. 1B is a top section view, illustrating the bulk materialirradiation system of the present invention

FIG. 2 is a graph illustrating depth-dose relationships for irradiationbeams having electrons with varying energy levels.

FIG. 3 is a graph illustrating a depth-dose relationship for two-sided1.8 MeV exposure of a product having a thickness of 1.5 cm. (or 0.6inches).

FIG. 4 is a diagram illustrating an electron beam acceleration andscanning system for providing two-sided irradiation to a product.

FIG. 5 is a diagram showing a first modified version of the two-sidedexposure system of FIG. 4.

FIG. 6 is a diagram showing a second modified version of the system ofFIG. 4.

FIG. 7A is a diagram illustrating a typical scanning configuration forelectron beam devices.

FIG. 7B is a graph illustrating the relative dose delivered by thescanning configuration shown in FIG. 7A.

FIG. 8A is a diagram illustrating a typical electron beam scanning anddeflection system.

FIG. 8B is a diagram illustrating a scan horn configured in an obliqueconfiguration.

FIG. 9 is a schematic block diagram of control system 120 that providesthe capability to locate the beam spot position with improved linearprecision.

FIG. 10 is a diagram illustrating the increase in beam spot size whilemaintaining the center location of the beam spots according to anembodiment of the present invention.

FIG. 11 is a diagram of an exemplary material contact structure capableof providing an effective interface between adjacent scan horn and bulkmaterial tube structures.

FIG. 12 is a diagram of another exemplary material contact structurecapable of providing an effective interface between adjacent scan hornand bulk material tube structures.

FIG. 13 is a diagram illustrating an exemplary dosimetry carrier forinsertion into the material flow path according to the presentinvention.

FIG. 14 is a diagram illustrating an exemplary dosimetry carrier inputaccess port according to the present invention.

FIG. 15 is a diagram illustrating an exemplary dosimetry carrier exitport according to the present invention.

FIG. 16 is a diagram illustrating a configuration for two-sidedirradiation of a plurality of cylindrical tubes in a side-by-sidearrangement.

FIG. 17 is a diagram illustrating an exemplary laminated tubularmaterial exposure structure.

FIG. 18 is a diagram of another exemplary laminated tubular materialexposure structure.

FIG. 19 is a diagram illustrating a configuration for two-sidedirradiation of a plurality of cylindrical tubes in an alternating offsetarrangement.

FIG. 20 is a graph illustrating depth-dose curves for varyingthicknesses of material.

FIG. 21 is a diagram showing a bulk material tube having an exposurecompensated geometry.

FIG. 22 is a diagram of another embodiment of an exposure compensatedbulk material tube.

FIG. 23 is a diagram of an exposure compensated bulk material tubehaving a structure that is a combination of the structures of FIGS. 21and 22.

DETAILED DESCRIPTION

FIG. 1A is a front section view, and FIG. 1B is a top section view,illustrating bulk material irradiation system 10 of the presentinvention. As shown in FIG. 1A, bulk irradiation system 10 includesinput hopper 12, pump assembly 14, bulk material tube 16, irradiationmodule 18, accelerator assembly 19, scan horns 20 a and 20 b, magnetassemblies 22 a and 22 b, and dosimetry shuttle ports 24 a and 24 b.Input hopper 12 receives bulk material to be irradiated, such as freshground beef in an exemplary embodiment, and the bulk material is forcedby pump assembly 14 to flow through bulk material tube 16. In anexemplary embodiment, pump assembly 14 also removes oxygen from the bulkmaterial flowing through tube 16, to enhance the effects of irradiation.University research and industry experience has shown that irradiationof meats such as ground beef in the presence of ordinary concentrationsof oxygen can cause oxidation of lipids that results in degradation ofthe quality and consumer acceptance of the processed product. It hasbeen demonstrated that irradiation in a modified atmosphere thatexcludes oxygen during the irradiation process can eliminate thisundesirable effect.

Bulk material flows through tube 16 into irradiation module 18 at anupward angle, so that the walls of irradiation module 18 are able toprovide effective shielding from radiation that would otherwisepotentially exit irradiation module 18 and present a hazard to operatingpersonnel. Accelerator assembly 19 generates an electron beam or othercomparable irradiation beam that is directed through magnet assembly 22a and scan horn 20 a to irradiate bulk material flowing in tube 16 fromthe top side, and also is directed through magnet 22 b and scan horn 20b to irradiate bulk material flowing in tube 16 from the bottom side.Although double-sided irradiation is shown in the exemplary embodimentof FIG. 1A as being provided by a single accelerator and selectivelydirected in two irradiation beam paths, it should be understood by thoseskilled in the art that dual accelerators could be employed with similareffect. In addition, although the exemplary embodiment of FIG. 1A showsdouble-sided irradiation of bulk material flowing in tube 16, it shouldbe understood by those skilled in the art that single-sided irradiationmay be employed where sufficiently small depths of penetration arearequired and/or where the energy of the electron beam generated byaccelerator assembly 19 is sufficiently high to achieve the necessarypenetration. A further discussion of single-sided and double-sidedirradiation will occur below with respect to FIGS. 2 and 3. After bulkmaterial flowing through tube 16 has been irradiated, the material exitsirradiation module 18 at an upward angle, again so that the walls ofirradiation module 18 are able to provide effective shielding fromradiation that would otherwise potentially exit irradiation module 18and present a hazard to operating personnel.

Dosimetry shuttle port 24 a is provided in bulk material tube 16 betweeninput hopper 12 and irradiation module 18, allowing the insertion of adosimetry module into the flow of bulk material in tube 16. Thedosimetry module is therefore able to pass through irradiation module 18in tube 16, and receive irradiation from scan horns 20 a and 20 b. Thedosimetry module can then be ejected from tube 16 through dosimetryshuttle port 26 a on the opposite side of irradiation module 18, foranalysis and calibration of the irradiation dose delivered by thesystem.

FIG. 1B is a top section view of bulk material irradiation system 10,with a few modifications according to another exemplary embodiment ofthe invention. FIG. 1B illustrates an embodiment having two inputhoppers 12 a and 12 b, two pump assemblies 14 a and 14 b, and two bulkmaterial tubes 16 a and 16 b passing though irradiation module 18.Dosimetry shuttle ports 24 a and 24 b are provided in bulk material tube16 a, and dosimetry shuttle ports 24 c and 24 d are provided in bulkmaterial tube 16 b. It should be understood by those skilled in the artthat any number of bulk material tubes such as tubes 16 a and 16 b maybe employed to implement bulk material irradiation system 10 of thepresent invention, with one or more accelerator assemblies 19 andrelated magnets and scan horns. A system is preferably designed toachieve a desired rate of bulk material throughput while also ensuringthat proper irradiation doses are received, employing an appropriatenumber of bulk material flow paths and irradiation sources to accomplishthis result. More details of exemplary bulk material flow patharrangements will be discussed later.

The beneficial effects of irradiation of food are caused by theabsorption of ionizing energy that result in the breaking of a smallpercentage of the molecular bonds of molecules in the product. Most ofthe molecules in food are relatively small and are therefore unaffected.The DNA in bacteria, however, is a very large molecule and is highlylikely to be broken and rendered unable to replicate. The absorption ofradiation in the food product causes the radiation intensity to bereduced according to a depth-dose relationship that is scientificallywell known.

FIG. 2 is a graph illustrating depth-dose relationships for irradiationbeams having electrons with varying energy levels. Curve 30 representsthe depth-dose relationship for 1.8 MeV electrons, curve 32 representsthe depth-dose relationship for 4.7 MeV electrons, and curve 34represents the depth-dose relationship for 10.6 MeV electrons. As shownby curves 30, 32 and 34, the radiation intensity actually increases to amaximum at a distance somewhat interior to the surface of the productbeing irradiated due to scatter emission of radiation from electroncollisions with food molecules. After the maximum is achieved,absorption causes the relative intensity to begin to fall off untilvirtually all of the radiation has been absorbed. At the “tails” of thedepth-dose curves the intensity is much less than the maximum, but stillresults in an incremental amount of beneficial irradiation. Single sidedapplication of radiation that is required to maintain a moderate ratiobetween maximum and minimum exposure must necessarily waste most of thistail of radiation intensity. Curve 30 for example illustrates that thepercentage dose is at approximately 50% of the maximum value at apenetration depth of about 0.7 cm (or 0.28 inches). Exposure of food ofthis thickness would result in a maximum/minimum ratio of 1/0.5=2.0,while approximately one-third of the beam power would pass through thematerial and be wasted. While single-sided irradiation can deliverproper doses of irradiation to product, the potential waste of power andlimited depth of product that may be effectively irradiated can limitthe effectiveness of the irradiation system.

A solution to this waste and inefficiency problem is to expose theproduct to the electron beam from two sides. FIG. 3 is a graphillustrating a depth-dose relationship for two-sided 1.8 MeV exposure ofa product having a thickness of 1.5 cm. (or 0.6 inches), shown by curve40. The depth of effective irradiation is substantially greater than thesingle sided exposure and the maximum/minimum ration is substantiallylower resulting in more precise and consistent product exposure. Whiletwo-sided irradiation is preferred for maximum efficiency and mostconsistent exposure, generation of the two sided radiation sources addscomplexity. The typical solutions are to either pass product through theradiation source once per side which requires twice as long to process,or to create two independent accelerators which is effective but can becostly and complicated.

Food may be irradiated at a number of points during processing anddistribution depending on the product shape, thickness and packaging.Foods that are relatively thick require relatively high energy radiationexposure for consistent penetration to the interior of the product, forexample, two sided 10 MeV electron beam exposure is necessary toirradiate meat that is up to 3.5 inches thick. Foods that are not asthick, however, may be effectively irradiated by much lower energysources, for example, products that are 0.6 inches thick may beirradiated by two-sided 1.8 MeV electron beam exposure. The advantagesof the lower electron beam energy are that a less complex acceleratorsystem may be used to generate the beam, and the shielding requirementsare not as great due to the reduced penetration of the beam. The primarychallenge for lower energy irradiation systems is material handling toaccurately prepare, present and manage the product exposure. All foodirradiation systems have the objective of reducing the levels of harmfulfood borne pathogens. To ensure that no cross contamination orrecontamination of irradiated food by any other pathogens can occur, itis necessary that food either be irradiated in final pathogenimpermeable packaging, or be maintained in an environment thateliminates the recontamination potential.

A number of sources are potentially available for generating ionizingradiation that is able to effectively irradiate product. Some sourcesthat may be used include gamma sources, high energy electrons and x-rayradiation. The use of these sources for irradiation is generally knownin the art. The present invention will hereafter be described as itpertains to the use of a linear accelerator for producing a beam of highenergy electrons for irradiation.

FIG. 4 is a diagram illustrating an electron beam acceleration andscanning system for providing two-sided irradiation to a product. Thesystem includes an accelerator having electron gun 50 and acceleratorwaveguide 52, upper magnet assembly 22 a including beam deflectionmagnet 54, upper quadrupole magnet 56 and upper scan magnet 58, upperscan horn 20 a, beam sensors 62, lower magnet assembly 22 b includingbending magnet 64, lower quadrupole magnet 66 and lower scan magnet 68,and lower scan horn 20 b. Material to be irradiated passes through area70 between upper scan horn 20 a and lower scan horn 20 b. Singleaccelerator 19 is shared between the upper exposure subsystem and thelower exposure subsystem. Beam deflection magnet 54 is controlled by anapplied current to either allow an accelerated electron beam fromaccelerator waveguide 52 to travel directly downward or to be deflectedsideways toward upper scan horn 20 a. If the beam is deflected towardupper scan horn 20 a, normal beam scanning operation occurs. Thisconsists of passing the beam through upper quadrupole magnet 56 to formthe beam spot to a larger elliptical shape, and to apply a currentthrough upper scan magnet 58 to deflect the beam over the traversalrange of upper scan horn 20 a. The angle-fed asymmetrical shape of upperscan horn 20 a will result in the beam spot position steps beingdifferent depending on the particular position of the beam. This stepposition variation may be eliminated by a computer controlled positionmanagement system which maps a physical spot location to a particularbeam deflection angle and applies an appropriate current to thedeflection magnet to locate the beam spot to that exact position. For atypical scan traversal range of 92 cm (or 36 inches) and a spot size oftypically 6×9 cm., a total of 32 position steps would be needed to fullycover the scan range. Each of these 32 steps would have associated acomputer stored scan magnet control value of typically 10 bits or 1024values that provides the resolution to locate that particular beam spotwith necessary precision. The spot size will be somewhat larger at theouter extremity of the scan traversal range, but since the spot locationincrements are all exactly the same, the result is increased overlap andexactly the same exposure as the near extremity.

If the beam is not deflected toward upper scan horn 20 a, it continuesdownward until it passes through sensors 62 to bending magnet 64.Sensors 62 are structures consisting of two pairs of parallel sensingplates that the electron beam passes through, generating a differentialvoltage if the beam is nearer to one of the plates than the other. Thisvoltage may be sensed and used to adjust the current in a small pair ofmagnets associated with upper beam deflection magnet 54 to veryaccurately steer the beam into the receiving region of lower bendingmagnet 64. Lower bending magnet 64 has a current flowing through it thatbends the beam upward toward lower scan horn 20 b. The operation of thelower scan subsystem is identical to the upper scan subsystem and thebeam is directed alternately between the two by the alternate control ofcurrent through deflection magnet 54 under computer control.

The upper and lower scan subsystems of FIG. 4 are offset so that theyare not located exactly opposite each other, which allows the placementof sensors opposite material plane area 70 that can be used toaccurately determine the actual location of the beam spot. These sensorsmay also be used to sense the presence of material under the scan hornsso that no scanning power is wasted when no material is present.

It is possible to selectively control the power of each successive pulsethat makes up an electron beam. A detailed disclosure of such dynamicpower control may be found in U.S. application Ser. No. 09/685,779 filedOct. 10, 2000 for “radiation System And Method” and assigned to Mitec,inc., the same assignee as the present application. The aforementionedU.S. application Ser. No. 09/685,779 is hereby incorporated by referencein its entirety. In an exemplary embodiment of the irradiation system ofthe present invention, a sensor may be provided on a side of bulkmaterial tube 16 opposite scan horn 20 a (and also on a side oppositescan horn 20 b) to measure a level of ionizing radiation delivered tothe bulk material. The power of each pulse of the electron beam may thenbe dynamically controlled based on the sensor measurements to maintain adesired dose of radiation delivered to the bulk material. In addition,pump assembly 14 maybe controlled to adjust the flow rate of bulkmaterial in tube 16 based on sensor measurements to further control theirradiation dosage delivered to the bulk material.

FIG. 5 is a diagram showing a modification of the two-sided exposuresystem of FIG. 4 with a pair of reduced size scan horns 20 a and 20 bplaced opposite a flattened tube shaped structure 16. Tube shapedstructure 16 is a closed pipe-like feeder that may receive materialthrough a pipe from a pump system (e.g., pump assembly 14, FIG. 1) thatforces material into a flattened shape to move it through irradiationscan horns 20 a and 20 b for irradiation exposure. This system mayhandle all types of liquid and many types of formable or soft materialssuch as ground meats that may be forced through tube 16 under pressure.The thickness of the material to be irradiated is determined by thethickness of the flattened tube structure 16. A feeder of 8 inches wideand 1.6 inches thick has approximately the same cross-sectional area asa circular pipe of 4 inches in diameter, which is a preferred size for amaterial pumping system.

FIG. 6 is a diagram showing a modified version of the system of FIG. 4,whereby combined upper deflection and scan magnet 74 is employed toperform the function of the scan magnets and the function of the upperdeflection magnet, and combined lower bending and scan magnet 76 isemployed to perform the function of the lower bending magnet and thefunction of the lower scan magnet. The configuration of FIG. 6 reducesthe number of magnets that are required for operation of the system. Ineither system, an embedded computer control system is employed to setthe current in the deflection and scan magnets and control thegeneration and timing of an irradiation pulse to precisely apply theexposure to sequentially stepped positions.

In a typical operational mode of the invention (as shown in FIG. 5, forexample), a computer directs an electron beam toward upper scan horn 20a by selecting a value of current in upper beam deflection magnet 54that bends the electrons the appropriate amount. Quadrupole magnet 56forms the beam to the specified elliptical shape and size, and thecomputer sets a predetermined current value in scan magnet 58 to directthe beam to the desired beam spot position and the accelerator is pulsedto irradiate the selected position. The computer then sets the next beamspot position by controlling the scan magnet current and the next pulseis applied with a nominal 50% overlap of the first beam spot. Once acomplete sweep of the beam spots is completed for the top side of thematerial feeder tube, the computer selects the appropriate current inthe beam deflection magnet to cause the beam to travel to the lowerbending magnet. In similar fashion, the computer controls the beamposition by selecting a current value in lower scan magnet 68, theaccelerator is pulsed, and the cycle continues until the entire lowerregion is irradiated by a complete sweep. By using this system, singleaccelerator 19 may be shared alternately between the upper and lowerscan horns to provide the preferred two-sided irradiation exposure.

FIG. 7A is a diagram illustrating a typical scanning configuration forelectron beam devices, and FIG. 7B is a graph illustrating the relativedose delivered by the scanning configuration shown in FIG. 7A. Anelectron beam is typically generated as a timed pulse of 10 to 15microseconds in duration with a repetition rate of typically 500 pulsesper second. The electron spot is directed by magnets toward the materialto be irradiated in sequentially overlapped positions with an overlap ofnominally 50% of the spot size to provide a uniform radiation exposure.Material is typically moved through the scanned spot region at a ratethat allows 50% overlap of the spot in the horizontal dimension as well.

An electron beam spot is formed to an elliptical shape of approximately3:2 width-to-height ratio with a horizontal spot size of typicallyseveral centimeters. The beam spot is positioned in a vertical dimensionby driving a current into a scanning electromagnet with an initialposition beginning nominally at the extremity of a total scan traversalrange which is indicated in FIG. 7A at location 80. A pulse ofaccelerated electrons is generated and applied to the selected physicallocation 80 to provide an irradiation dose to that physical position. Acontrol circuit then drives a new current magnitude into the scanelectromagnet to move the beam spot location to position 81 to overlapposition 80 by 50%, the pulse is generated and irradiation exposure isapplied to that position. The process continues until position 82 isselected, pulsed and irradiated. The control circuit then applies thecurrent associated with the original position 80 to the scanelectromagnet to quickly move the spot position back to the initialvertical position of location 80. During the time that the vertical scanfrom position 80 to 82 is progressing, the material is being movedhorizontally through the scan traversal range. If the material moves ata velocity that causes the horizontal displacement during this scan timeto be half of the width of the spot, then the actual position of thespot after position 82 will be at position 83, which results in 50%horizontal overlap of position 83 with position 80. This dual verticaland horizontal overlap results in uniform total exposure of the materialas illustrated in FIG. 7B. The vertical lines 94 and 95 indicate thewidth of the material that is moved through the scan traversal range.The total dose applied to the material is indicated by the curve 96 andis nearly constant over the area that the overlap is 50%. The doublevertical and horizontal overlap results in total average exposure equalto 4 times the individual beam spot exposure.

Prior art scanned and pulsed electron beam irradiation systems asdescribed in FIGS. 7A and 7B depend on the uniformity of the material tobe irradiated, invariant velocity of material movement, constantelectron beam power and precise positioning of the electron beam spot toachieve uniform dosage. If any of these parameters varies, the resultingdosage will be affected either upward or downward resulting either inpotential overexposure and material quality reduction, or in lesseffective reduction of the targeted harmful pathogens that theirradiation process is intended to achieve.

FIG. 8A is a diagram illustrating a typical electron beam scanning anddeflection system. Scan horn 20 a, typically consisting of an evacuatedmetal enclosure, provides an environment that accelerated electrons canpropagate through with minimal loss of energy. Scan horn 20 a issufficiently large that the electron beam spot with an elliptical shapeas described in FIG. 7A does not contact any of the metal walls of scanhorn 20 a.

Accelerated electrons are received from an accelerator in compactcylindrical beam 103 with a diameter of typically 0.5 cm. The compactcylindrical beam of electrons 103 is formed into the preferredelliptical spot shape 80 by quadrupole magnet 56 typically consisting ofa pair of electromagnets with ferromagnetic pole structures shaped toact on the electron beam in a manner analogous to an optical lens,whereby the amount of deflection of the electrons is proportional to theradial displacement of the electrons from the center of the beam. Theresult is that the beam spot intensity is spread into an ellipticalprofile wider than compact cylindrical shape 103 that is employed duringthe acceleration of the electrons to allow application of eachindividual pulse of the beam to a larger amount of material. If thismethod were not used, the relative power of the accelerator would haveto be reduced to avoid overexposure of material at each spot, and theresultant processing speed would be reduced.

Scanning electromagnet 58 receives the elliptically formed and spreadelectron beam spot 80 at the entrance to scan horn 20 a and deflectsbeam spot 80 in an angular amount proportional to the electromagnetcurrent. If the current steps applied to the scanning electromagnet 58increase in identical amounts, the deflection of the electron beam spotwill be also be an identical angle. The preferred maximum deflection isapproximately 20 degrees of arc in either direction from beam center105, as is indicated by rays 107 and 108 resulting in a total deflectionof approximately 40 degrees. The linear scan traversal range dictatesthe length of scan horn 20 a to maintain the total 40 degree beamdeflection. A scan horn with a scan traversal range of 75 cm wouldrequire a scan horn vertical length of 103 cm. Since the beam deflectionamounts caused by the application of constantly spaced current steps inscanning magnet 58 result in constantly spaced angles of deflection, itcan be seen that the linear displacement per current step at the centerof the scan horn exit region 105 is smaller than the linear displacementper current step at the maximum deflection regions 107 and 108. With abeam spot that is 6 cm tall, the preferred 50% overlap would require astep size of 3 cm. For a scan traversal range of 75 cm, this wouldtranslate to an angle of deflection from the center 106 of 1.6683degrees which would move the beam to position 106. Application ofconstant current steps that cause sequentially increased currents wouldcause sequentially increased deflection angles until the maximumdeflection 108 is reached. Since discrete steps are required, themaximum deflection amount would be 20 degrees divided by the step anglesof 1.6683 degrees, which would be 12 step values for a total angle of20.02 degrees. The next step inward 109 from the maximum deflection 108would be 11 step values for a total deflection of 18.352 degrees. Thelinear displacements 110 and 111 would be 37.54 and 34.18 cm.respectively, which would result in an overlap amount of 3.36 cm. or 44%overlap of the spots at the edge. The resulting irradiation exposurewould consequently be below the specified target value by 6% at theouter edges of the irradiation scan area.

Scan horn structures may be desired that employ deflection angles thatare within the 40 degree maximum total deflection, but that use anoblique structure that exaggerates the deflection angle amount at theouter extremities of the scan traversal range. FIG. 8B is a diagramillustrating scan horn 20 a configured in such an oblique configuration.Similar to the geometry at the extremities of the scan horn shown inFIG. 5A, the linear deflection is greater at the outer angle 114 thanthe inner angle 112. In the case of the oblique scan horn, thedifferential angle of deflection for the 3 cm step between 116 and 117with 50% spot overlap is the minimum at the inner beam position 112. Anoblique scan horn with a scan traversal range of 75 cm would require ascan horn vertical length of 89.38 cm. The deflection angle required for3 cm deflection from the inner angle 112 to the next angle 113 is 1.922degrees. A deflection to the outer deflection position 114 would require21 steps of 1.922 degrees for a total deflection of 40.36 degrees. Thedeflection at the 20th step 115 would be 38.44 degrees. The differencein the linear displacement at the outer extremity indicated by 119 and118 would be 5.02 cm., which would result in only 16% overlap. Theirradiation exposure would be below the specified target value by 40% atthe outer edge of the irradiation scan area between 115 and 114, absentsome sort of compensation scheme.

FIG. 9 is a schematic block diagram of control system 120 that providesthe capability to locate the beam spot position with improved linearprecision. Controller 121 is used to generate a digital control valuethat is output through digital interface 122 to digital to analogconverter 123. The output of digital to analog converter 123 is input tocurrent driver 124 that generates a continuously variable current outputthat is precisely proportional to the analog voltage input. This currentoutput is driven through scanning magnet 58 to deflect the electron beamby an amount proportional to the magnet current. The precision of thelocation of the electron beam spots is dependent on the precision of thedigital outputs of controller 121. An exemplary embodiment of controller121 is a digital computer with digital precision of 16 bits or more.This digital precision allows each beam spot position to be individuallylocated by outputting a scan magnet deflection current that correspondsto a constant increment in linear displacement.

In the offset scan horn example of FIG. 8B, it is desired that eachincrement of the scan spot location be spaced by an amount of 3 cm.Table 1 shows the deflection angles that result in an incremental spotlocation of 3 cm. 12 bits of digital precision are sufficient to resolvea total of 4096 steps of current value. A 12 bit digital to analogconverter with a control value input as indicated in Table 1 will resultin the “X Actual” linear displacement. The “Error %” is an indication ofthe difference between the “X Actual” position and the exact “Xdistance” position that would result from 3 cm. incremental distance. Ascan be seen, the maximum error is less than 0.4%, which will result invery consistent and precise exposure control, especially in the case ofan offset scan horn such as the type shown in FIG. 8B.

TABLE 1 Step Angle X distance Control Value X Actual Error % 1 0.00000.0000 0 0 0.0000 2 1.9106 3.0000 191 2.9990 0.0334 3 3.8170 6.0000 3826.0047 −0.1558 4 5.7150 9.0000 572 9.0079 −0.2638 5 7.6005 12.0000 76011.9992 0.0259 6 9.4696 15.0000 947 15.0007 −0.0240 7 11.3185 18.00001132 18.0024 −0.0811 8 13.1439 21.0000 1314 20.9936 0.2147 9 14.942524.0000 1494 23.9958 0.1412 10 16.7115 27.0000 1671 26.9974 0.0866 1118.4483 30.0000 1845 30.0029 −0.0973 12 20.1507 33.0000 2015 32.99870.0425 13 21.8168 36.0000 2182 36.0059 −0.1951 14 23.4450 39.0000 234438.9907 0.3084 15 25.0340 42.0000 2503 41.9924 0.2537 16 26.5829 45.00002658 44.9943 0.1889 17 28.0910 48.0000 2809 47.9980 0.0674 18 29.557951.0000 2956 51.0043 −0.1446 19 30.9834 54.0000 3098 53.9927 0.2440 2032.3676 57.0000 3237 57.0053 −0.1761 21 33.7106 60.0000 3371 59.99850.0489 22 35.0130 63.0000 3501 62.9931 0.2314 23 36.2751 66.0000 362866.0118 −0.3945 24 37.4977 69.0000 3750 69.0057 −0.1905 25 38.681672.0000 3868 71.9960 0.1337 26 39.8275 75.0000 3983 75.0067 −0.2220 2740.9364 78.0000 4094 78.0098 −0.3256

The offset scan horn shown in FIG. 8B coupled with precision deflectioncontrol system 120 of FIG. 9 makes it possible to locate the center ofthe beam spot very accurately as indicated in Table 1. The same radialspreading of the linear position of the center of the beam at the outerextremities of the scan traversal range will also cause the beam spotsize to be increased in both vertical and horizontal directions. Whilethis radial spreading of the beam spot center can cause exposure error,the increase in the size of the beam spot does not cause error so longas the center location of the beam spot is compensated and linearizedaccording to the method illustrated in FIG. 9 and Table 1. FIG. 10 showsthis beam spot size increase in somewhat exaggerated form to illustratethe concept. Since the center of each beam spot position is located atexactly the same linear displacement, the affect of the beam spot sizeincrease is to simply increase the overlap of the exposure. Position 126has exactly the same center position (and size, in this case) asposition 80 of FIG. 7A. Position 127 is located at exactly the samecenter position as position 82 of FIG. 7A. Since identically the sameamount of irradiation exposure is applied to the total area andthickness, the exposure is also identically the same. Thus, consistentdosages can be delivered throughout an entire scan traversal range as aresult of the present invention.

In irradiation systems such as have been described above with respect toFIGS. 5 and 6, for example, material must move through flattened tube 16as it is exposed to radiation from scan horns 20 a and 20 b. Since thereis pressure inside flattened tube 16, and potentially substantial solidmaterial to be irradiated, flattened tube 16 must be durable enough towithstand the pressure. At the same time, it is necessary for radiationto pass through flattened tube 16 with as little attenuation as possibleso that it may effectively irradiate the product passing through. Thisgenerally requires that the material through which radiation passes mustbe very thin. The material used in typical scan horns such as the typeshown in FIG. 4 is titanium foil having a thickness of approximately0.005 inches, which typically serves as a barrier only between the highvacuum conditions inside the scan horn and the atmosphere outside thescan horn. In the case of a material-to-scan horn barrier such as shownin FIGS. 5 and 6, a 0.005 inch foil would potentially be too fragile tomaintain high vacuum conditions on one side and pressurized solidmaterial on the other side.

FIGS. 11 and 12 are diagrams illustrating material contact structurescapable of providing an effective interface between adjacent scan hornand bulk material tube structures. FIGS. 11 and 12 show scanhorn-to-material interface structure 130 consisting of a mating adapterthat provides isolation between scan horn 20 a and bulk material tube16. This isolation barrier performs several functions. First, tominimize beam power attenuation, the interface between the material tobe irradiated and the scan horn must be as thin as possible whilemaintaining sufficient strength to contain the pressurized material intube 16 without perforation. This interface includes interface 132between scan horn 20 a and the mating adapter and interface 134 betweenthe mating adapter and tube 16. A foil material of between 0.020 and0.040 inches may be used as the irradiation barrier window with beamattenuation proportional to the foil thickness. While this thickness ishigh by electron beam exit window standards, it is quite thin bymaterial handling standards, and would typically fail quickly inpressurized operation. The mechanical integrity of the foil can beenhanced, however, by supporting the opposite side of the materialinterface foil with gas flowing in path 136, pressurized to a levelsimilar to the pressure inside bulk material tube 16. This gas may beair or some other selected gas appropriate to this function. The gaspressure may be statically controlled, or there may be a pressure sensorplaced inside tube 16 that determines the pressure on the material sideof the foil and controls the gas pressure on the opposite side of thefoil to maintain zero differential pressure across the foil. Such apressure management system will minimize flexing of the foil andresulting cracking and failure. The scan horn exit foil may need to besomewhat thicker to withstand the larger pressure differential betweenvacuum and the pressure typical of the interior the feeder tube.

Both the thin foil that maintains the vacuum barrier for the scan hornand the thicker material contact foil will absorb power and will beheated by the electron beam as the beam passes through. To insure thatthe foils do not overheat and fail, it is necessary to provide a coolingsystem. The pressurized gas in the interface structure may be pumpedthrough the interface cavity at a moderately high volume to providingcooling for the foils. The pressurized gas may also be chilled toprovide greater heat transfer from the foils to the gas if needed.

A typically preferred material for electron beam exit window foil istitanium due to its high strength and relatively low electron beamattenuation. Titanium is also an acceptable material for food contact,suggesting that bulk material tube 16 may be composed of titanium in anexemplary embodiment. However, in some applications stainless steel maybe a preferred material for food contact. Stainless steel has higherbeam attenuation and is therefore not as suitable for electron beamtransmission. A solution to these conflicting requirements is toconstruct a food contact foil by laminating a sheet of titanium with asheet of stainless steel on the food contact side. The stainless steelsheet may be very thin to provide food contact with minimum beamattenuation, while the titanium may make up most of the remainingthickness to maximize strength with minimum attenuation.

FIG. 12 is a diagram illustrating an exemplary construction of the scanhorn-to-tube interface according to the present invention. High energyelectrons are passed through bulk material flowing through tube 16 viaelectron beam access window 149, which as mentioned above may betitanium foil, or may alternatively be a lamination of a relatively thinsheet of stainless steel of 0.002 to 0.004 in. thick for conformancewith a particular food handling methodology and a relatively thickersheet of titanium for combined strength with minimum attenuation of theelectron beam energy. The foil may be on the order of 0.020 to 0.040 in.thick and is by itself too fragile to withstand the pressure required toforce various liquid or formable materials through a tube of as long asten feet or more. The relatively fragile foil is reinforced by applyingpressurized gas on the opposite side of the foil and maintaining the gasat exactly the same pressure as the material inside the tube to hold theforces on the foil in equilibrium, as described above with respect toFIG. 11. The gas pressure applied to the back side of the foil iscontrolled by measuring the pressure applied to the material inside thetube and providing a feedback control signal to an electronicallycontrolled gas feed valve. Gas may flow in and out at some sustainedrate while maintaining the balancing pressure to conduct heat away fromthe foils that is caused by electron absorption.

Optimum movement of material through the tubular material handlingsystem depends on the path for the material being as smooth andtopologically consistent as possible. Ideally, once material such asground meat is formed to the shape of the flattened tube, this shapeshould be maintained through the entire process. This is of particularimportance in the area of electron exposure foil 149, where the materialmust be consistent in thickness and velocity to maintain a uniformapplied irradiation dosage. An important feature of the structure shownin FIG. 12 is the uniformity of the shape of the inside of tube 16. Thebenefits of this uniformity are that the pressure required to cause thematerial to move is minimized, the flow rate of the material across thewidth of the tube is constant and as will be described in more detail,it is possible to pass a carrier device with exterior dimensionsmatching the interior dimensions of flattened tube 16 through tube 16 ata rate equal to the material flow rate.

Tube 16 is typically in the shape of a flattened pipe. The structure hasflanges 141 at each end for connection to other sections of the materialhandling system. Mating structure 142 is positioned in an access hole inthe side of tube 16 and is held in place by bolts 143. Mating structure142 is further fastened to scan horn 20 a which directs electron beamspot 146 in vacuum toward thin metal foil 147 which serves as theinterface between the high vacuum required for electron beamacceleration and pressurized volume 148 that supports material contactlaminated foil 149. Material contact foil 149 is supported by rigidstainless steel frame 150 that is fastened into place by bolts 151 andsecured to mating structure 142. Material contact foil 149 should bequite thin to allow for efficient transmission of electrons through thematerial, but it must also be capable of sustaining the pressure andpotential deformation that will result from contact with liquid or solidmaterials. With pressurized gas in region 148 maintained on the backside of material contact foil 149, the stress applied to materialcontact foil 149 is greatly reduced.

To allow for smooth movement of material through the electron exposurestructure, material contact foil 149 and its carrier must be designedand fabricated to fit mating structure 142 precisely with no gaps orcrevices that might allow for food material to become lodged within.Carrier frame 150 may be fabricated of relatively rigid stainless steelmaterial with dimensions that mate precisely to the surfaces of matingstructure 142. Foil 149 may be bonded to stainless steel carrier frame150 by welding for maximum strength and mechanical integrity. Bolts 151and nuts 143 may be fabricated as studs bonded to carrier frame 150 andtube 16, respectively.

A basic requirement of irradiation systems in general and foodirradiation systems in particular is establishment and calibration ofthe irradiation dose that is applied to materials. Prior art irradiationsystems typically apply radiation to products that have been processedand packaged as individual items of some type. Verification of thedosimetry for such prior art systems typically involves positioning anumber of dosimeters at various places on and within the products to beirradiated, and measuring the dose applied to those dosimeters byprocessing. This verification is required by government regulatoryagencies to insure compliance with the established processingguidelines. It is further required that the dosimetry verification betraceable to a calibrated standard maintained by a government standardsuch as the National Institute for Standards and Technology (NIST).

The dose applied to bulk material pumped through an exposure tube may bevaried and controlled by several means. Material pumps are relativelyprecise in their ability to maintain pressure and volume, and may becontrolled by an electronic system to supply material through the tubeat a relatively constant rate. A more precise method is to utilize areasonably well controlled pump rate coupled with a precision materialvelocity measurement system that determines the actual rate thatmaterial is moving through the tube and slaves the irradiation exposurecontrol system to this actual flow rate. This method is particularlyuseful for maintaining accurate applied dose while the system isstarting or stopping whereby the flow rate of the material is reachingthe desired target speed or slowing down to a stop.

While pump assembly 14 is shown as an exemplary device for pressurizingbulk material tube 16 to force bulk material to flow through tube 16, itshould be understood that other mechanisms may be used to cause bulkmaterial to flow through tube 16. For example, the irradiation systemmay be constructed in such a manner that gravity is effective to causebulk material to flow “downhill” through tube 16. Other modifiedpressurization assemblies will be apparent to those skilled in the art.

Calibration and verification of dosimetry for bulk material irradiationsystems such as the type described herein must employ differentmethodology than prior art systems since there are no individual itemsthat dosimeters may be placed upon or within. In either type of system,dosimeters are placed at the appropriate locations within the materialbeing irradiated so that exposure can be verified. For bulk materialirradiation, the dosimeters must be introduced into the material stream,positioned at the appropriate locations in the material stream andretrieved for verification measurement.

FIG. 13 is a diagram illustrating an exemplary dosimetry carrier 160 forinsertion into the material flow path according to the presentinvention. Dosimetry carrier 160 is formed from a flexible solid plasticmaterial suitable for food contact. The dimensions of carrier 160 areselected to fit snugly within the interior cross section of tube 16 sothat with either liquid or solid material, carrier 160 will be driventhrough tube 16 by the flow of the material. Ordinary alanine chemicaldosimeters of either pellet or strip form may be placed in preparedlocations 161-169 or other locations of interest for dose mapping of theirradiation exposure across bulk material tube 16. Carrier 160 holdseach dosimeter at an exact vertical and horizontal position and carrier160 is inserted into the material stream to be moved at exactly the samespeed as the material that is pumped through tube 16 for irradiationexposure. As was discussed in the description of the electron exposurestructure, it is important that the shape of the interior cross sectionof tube 16 be constant from entry to exit of the system so that thepressure required to force material through tube 16 is minimized anddosimetry carrier 160 will not be impeded in its travel through thesystem.

Dosimetry carrier 160 must be inserted into the material stream, exposedby the irradiation system, and retrieved from the material stream tomeasure the dosimetric accuracy of the system. Carrier 160 may beinserted into the material stream by the use of input access port 24 aas shown schematically in FIG. 14. Access port 24 a includes input tube171, output tube 172 and carrying module 173. In the normal mode, module173 is in the upper position whereby material flow passes through inputtube 171 through the lower port of module 173 and out through outputtube 172. In this mode, the upper channel of module 173 is open andfreely accessible to insert dosimetry carrier 160 into the channel. Oncedosimetry carrier 160 is in place in the module channel, module 173maybe quickly moved from its upper to its lower position by asufficiently powerful linear actuator such as pneumatic cylinder 174 ato cause carrier 160 to be placed in line with the material flow. Thismovement of the module 173 may be accomplished while the pumping systemis operational with a minimal amount of spillage of material, or thepump may be stopped and restarted after dosimetry carrier 160 isinserted and positioned into the material flow path.

Retrieval of dosimetry carrier 160 may be accomplished by usingextraction port 24 b shown in FIG. 15. Extraction port 24 b has astructure similar to input access port 24 a. Extraction port 24 b isalso a slide structure that is moved quickly by a similar linearactuator such as a pneumatic cylinder 174 b. The normal material flowposition is with the actuator in the lower position so that the materialmovement channel is routed from the irradiation module through portentrance 175 to port exit 176. When the actuator is lifted to its upperposition, the slide is moved up and the path from the irradiation modulethrough port entrance 175 is routed through waste gate port 177. Whendosimetry carrier 160 has been passed through the irradiation module, itmust be retrieved to measure the alanine dosimeters for dosagemeasurement. Carrier 160 is ejected from the material stream byexamining material flow measurements which accurately determine thelocation of the carrier and control the movement of waste gate port 177to minimize the amount of material that is ejected with carrier 160. Afurther utility of the waste gate structure is a fail safe mechanism toroute material out of the processed material path in the event of sometype of serious system malfunction which would result in unexposedmaterial passing through the system. Waste gate 177 provides a sealagainst the processed material flow past the gate so that cleanout canbe accomplished without completely purging the processed material fromport exit 906 and the associated downstream material handling path.

As described earlier, it is important that the exposure applied tomaterial to be irradiated be carefully calibrated and controlled to meetthe requirements of the regulatory agencies as well as maintainingacceptable product quality. The bulk tube fed system as described mustmanage irradiation dosage by applying the electron beam exposure to bulkmaterial as it flows through a flattened tube structure. Since the rateof flow of material through the tube is dependent upon pressure createdby a pump, the flow rate is not as constant as a material conveyor oftypical prior art systems. This somewhat variable material flow rate isaccommodated by the use of a material flow rate sensor that is placed inthe material input structure after the output of the input access port172. After the material has been formed to the flattened tubular shapeof the cross section of the exposure window structure, the flow rate isconsistent across the width of the material. The exposure of theelectron beam irradiation system is coupled by computer control to thematerial flow rate sensor so that the dosage is controlled in closedloop fashion as the material moves, no matter what its speed, up to themaximum flow rate.

Since the material to be irradiated is handled in bulk form with thissystem, it is important to insure that the irradiation dosage isreliably applied to all of the material that passes through the systemso that no possible unprocessed material is mistakenly passed throughthe system and assumed to be safe when it is not. One possible scenariothat must be mitigated is power failure that causes the accelerator tomomentarily pause generation of the electron beam, while material mightcontinue to flow through the system. This condition may be managed inone of several ways. The first and preferred method is to power theirradiation system through an uninterruptible power supply with aslittle as 30 seconds of power backup time. If main power is temporarilylost, this is sufficient time to stop the material flow pump and holdthe material stationary under the scan horns. If the power interruptionis temporary, the accelerator may be restored and the pump may berestarted. Since the accelerator exposure is slaved to the material flowrate, the material will receive a carefully controlled dosage even whileit is starting from a stationary condition.

The second and more drastic condition is an extended power outage, inwhich case the system will be stopped for a longer period of time, inwhich case the typical daily clean out and wash down procedure must beperformed. As a fail safe procedure, if some type of serious systemfailure is detected, the output waste gate can immediately gate productof unknown status out of the processed product flow to insure that thereis no mixing of processed and unprocessed product.

As has been discussed above, two-sided irradiation exposure is typicallyboth more efficient and provides more consistent exposure than singlesided exposure, and will be assumed to be the radiation source employedfor the following description of the present invention. Bulk materialmay be exposed to the two-sided irradiation source using an exposuresystem as shown schematically in FIG. 16. Bulk product may be movedthrough one or more cylindrical tubes 16 a-16 d in a side-by-sidearrangement 186, each having a diameter 184 small enough so that thedepth-dose curve of FIG. 2 generally governs the exposure. Although FIG.2 illustrates the two-sided depth-dose curve for 1.8 MeV electrons whichresults in a maximum exposure depth of 1.5 cm (or 0.6 inches), themaximum irradiation energy allowed by USDA and FDA rules is 10 MeV whichresults in a maximum exposure depth of 8.9 cm (or 3.5 inches). Thedepth-dose curve for 10 MeV electrons has the same general shape asdepth-dose curve 40 shown in FIG. 2. Cylindrical tubes 16 a-16 d serveto contain bulk material such as liquids or solids that may be pumpedthrough a pipe, such as ground beef. For ionizing radiation from upperelectron beam 180 and lower electron beam 182 to pass through tubes 16a-16 d, they must be constructed of material that is relativelytransparent to such radiation yet is sufficiently strong to contain thematerial within under pressure. The preferred material for suchapplications is titanium which exhibits both of these characteristics.Titanium has better radiation propagation characteristics than othermaterials, but absorption losses will still occur and must be minimized.An exemplary thickness for the titanium tubing in the radiation exposureregion is on the order of 0.010 to 0.020 inches, which is quite thin bynormal material handling structure standards. This unusually thinstructure is only required of the exposure portion of the structure,which may be coupled to an ordinary material handling tube of moretypical wall thickness of 0.100 to 0.250 inches. Cylindrical tubes 16a-16 d are one preferred shape for containment of bulk material underpressure, since the tube walls will be subjected primarily to tensilestress. Elliptical tubes may also be employed in another embodiment, forexample.

Titanium is preferred for radiation propagation through the barriermetal, and may also be used for food contact, but may not be a preferredmaterial for food contact in some applications. Other materials such asceramics and certain metals may be preferred for some food contactapplications. Stainless steel is a possible food contact material,however stainless steel is not a good material for propagatingradiation. FIG. 17 illustrates an exemplary tubular material exposurestructure 16 that comprises a lamination of a thin layer of stainlesssteel 192 on the interior for food contact bonded to an outer layer oftitanium metal 190 which provides the majority of the strength andphysical structure of the tube. The inner stainless steel layer 192 maybe 0.001 to 0.003 inches thick, while the outer titanium layer 190 maybe 0.008 to 0.019 inches thick depending on the internal pressure thatmust be sustained. The lamination may be constructed by plating a layerof stainless steel 192 or other food contact metal on the interior of athin seamless titanium tube 190.

An alternate tube construction method is to form tube 16 from flatlaminated sheets into circular sections that may be soldered, welded orclamped together to form a cylindrical tube shape 16 as shown in FIG.18. For example, a flat laminated sheet may include a thin layer ofstainless steel 192 and a thicker layer of titanium metal 192. Two suchsheets facing each other maybe shaped and soldered, welded or clampedtogether to form tube 16.

FIG. 19 is a diagram of another exemplary embodiment of theconfiguration of bulk material tubes 16 a-16 d according to the presentinvention. Tubes 16 a-16 d are configured in an offset arrangement 210to recover a portion of the irradiation beam power from upper electronbeam 180 and lower electron beam 182 that completely penetrates throughthe thinner cross sections at the edges of the tubes. With thisconfiguration, the total beam scan distance may be reduced and wastedbeam power will be minimized.

Multiple tubes 16 a-16 d are positioned adjacent to each other andlocated adjacent to the scan horn pairs to allow irradiation of multiplesimultaneous product streams that are physically separated from eachother. The irradiation control system may be controlled separately sothat each tube receives an independently set exposure level depending onthe physical location of the tube. A closed loop sensor device may beplaced below the tube opposite the scan horn to measure the dose that isapplied and adjust accordingly to maintain the minimum dosage at thelocations where the thickness is lower.

The thin tubular exposure structure maybe connected to an ordinary thickwall stainless steel food contact pipe for connection to the exterior ofthe irradiation system. The diameter 184 of the all of the tubular pipes16 a-16 d should preferably be constant to minimize material flowrestriction and pressure increase and to facilitate the movement of adosimetry calibration shuttle through the tube. A specialized foodmaterial pump (e.g., pump assembly 14, FIG. 1A) is used to developpressure to cause the material to flow through the material handlingtubes. Such pumps are typically constructed of stainless steel and arespecially designed to pump liquids and formable solids such as groundbeef without damaging the food material. Versions of these food materialpumps are available with vacuum pumps that remove up to 99% of theatmospheric air surrounding materials such as ground beef before thematerial is pressurized and directed out of the pump and into the outputtube. The oxygen content in the remaining 1% atmospheric air may befurther reduced by filling the input hopper structure of the pump withnitrogen or carbon dioxide gas. This process displaces oxygen from theatmospheric air to be subsequently removed by the vacuum pump. Thedescribed pump system coupled to the material handling tubes of thepresent invention serves to exclude virtually all of the oxygen frommaterials such as ground beef as a process step in the irradiation offresh ground beef. The removal of oxygen from the irradiationenvironment significantly improves the quality of irradiated fresh meatsuch as ground beef by preventing lipid oxidation.

While the cylindrical shape is preferred for physical strength andstructure, it is problematic for irradiation dose exposure managementdue to the phenomenon illustrated in FIG. 20, which is a graph ofdepth-dose curves for varying thicknesses of material. At the fullthickness of the cylinder, the depth-dose curve of FIG. 2 prevails andgood, consistent exposure results. This depth-dose curve is shown ascurve 238 of FIG. 20. As the exposure point is moved toward the edge ofthe cylinder and the effective thickness diminishes, the depth-dosecurves at the left region of FIG. 20 begin to be observed, and thedosage consistency is reduced to the point of being unacceptable. Curve220 illustrates the depth-dose relationship at a thickness of 0.8 cm,curve 222 illustrates the depth-dose relationship at a thickness of 1.8cm, curve 224 illustrates the depth-dose relationship at a thickness of2.6 cm, curve 226 illustrates the depth-dose relationship at a thicknessof 3.6 cm, curve 228 illustrates the depth-dose relationship at athickness of 4.4 cm, curve 230 illustrates the depth-dose relationshipat a thickness of 5.4 cm, curve 232 illustrates the depth-doserelationship at a thickness of 6.2 cm, curve 234 illustrates thedepth-dose relationship at a thickness of 7.2 cm, curve 236 illustratesthe depth-dose relationship at a thickness of 8.2 cm, and curve 238illustrates the depth-dose relationship at the full thickness of 9.0 cmIt may be noted that an elliptical shaped tube improves the amount ofthe total cross section that is near the preferred full thickness, butthere is still an amount of material that is near the edges of theelliptical cross section that receives the excessive dose as illustratedin FIG. 20.

This dilemma may be solved by utilizing an exposure compensated tubeconstruction in accordance with the present invention. The typicalobjective of material handling components in irradiation systems is tobe as efficient as possible. In particular, it is expected that thematerial handling components will have minimal interference with orattenuation of the electron beam. The exposure compensated tubeconstruction of the present invention is quite different from thistypical methodology, in that a material handling tube structure isdesigned to attenuate the electron beam according to a predeterminedcriterion. In particular, the criterion is for the total beam absorptionto be equivalent to that caused by a constant thickness sheet of groundbeef. The total absorption will be a combination of the absorption dueto ground beef summed with the absorption of a relatively thick crosssection of tubing of titanium or some other suitable material. With suchan exposure compensated tube, the thickness of the tubing increases asthe thickness of the material contained within it decreases. Formaterial such as titanium, the absorption of 10 MeV electrons isapproximately 300% greater than ground beef, so an amount of titanium ⅓as thick as the equivalent ground beef thickness will absorb an equalamount of beam power. The shape of the outer surface of the tube isdetermined by forming a tube material thickness at each point across thewidth of the tube corresponding to an absorption equal to the differencebetween the maximum thickness and the actual ground beef thickness atthat point. The effect is to create a composite absorption structurethat has a constant absorption corresponding to material of constantuniform thickness.

FIG. 21 is a diagram showing bulk material tube 16 having an exposurecompensated geometry. The interior shape may be any smooth profile,although an elliptical shape is generally preferred. The thicknesses oftube wall 250 at the edges are chosen for structural integrity,particularly at the top and bottom where beam absorption limits themaximum material thickness that may be processed. At the left and rightedges, most of the beam power will be absorbed by the material of tubewall 250. It will typically be necessary to provide air or liquidcooling to the tube in these areas to minimize heat buildup. If liquidcooling is employed, the absorption of the liquid should be accountedfor in the total absorption profile at the edge of the tube.

The modified tube cross section profile shown in FIG. 21 is used tocompensate for the reduced material thickness at the edges of the tube.According to an exemplary embodiment, the tube may be constructed ofsolid titanium material that is generally recognized as safe for foodcontact to directly contain bulk material such as ground beef that maybeforced through the tube by pumping under pressure. The increasinglythick cross section of the tube at the edges will absorb a substantialamount of the electron beam power to the point that at the outerextremity of the tube, all of the electron beam power will be absorbedby the tube. An approximation of the amount of power that will beabsorbed by the tube may be made by comparing the absorption that wouldoccur if the material being processed were rectangular and subtractingthe cross section area of the elliptical shape of the material. Theresult is

πr ²/(2r)²=π/4=0.785  (Eq. 1)

indicating that 78.5% of the power will be absorbed by the materialbeing processed, while 21.5% will be absorbed by the compensating tubestructure. A typical industrial irradiation system with a total beampower of 10,000 watts would cause 2,150 watts of power to be absorbed inthe compensating tube structure. The 7,850 watts of power absorbed bythe material being processed does not result in substantial productheating due to the fact that the power is applied to product that ismoving quickly through the exposure module. The 2,150 watts absorbed bythe tube structure, however, is persistently applied to the same area,so a significant amount of heating will occur. If this heat is notremoved continuously, the temperature of the exposure module will riseto the point that the product contained within could be damaged.

FIG. 22 is a diagram of an alternate embodiment of an exposurecompensated bulk material tube 16. The interior cross section of tube 16is still formed in an elliptical shape, however, the outer shape of tube16 is generally rectangular. The cross section between outside wall 255of the elliptical shape and the exterior rectangular shape is filledwith water 256 that is pumped through this area in a continuous flow toaccomplish two beneficial results. First, the water has the same basicirradiation absorption characteristic as meat or other products that maybe processed by irradiation, so forming a constant thickness of eitherthe water or product results in an equivalent compensation effect. Thenet effect is that the product and the water receive the same exposureprofile as the center portion of the product. Second, the water canremove heat from tube 16. Various types of product such as ground beefthat may be moved through tube 16 by pumping are more effectively movedif the temperature of tube 16 is controlled at a preferred level.

Walls 255 of the elliptical portion of tube 16 may constructed of thintitanium material to minimize the absorption of radiation as it isdelivered to the product being processed. Titanium is a rather poorconductor of heat, so being surrounded by temperature controlled wateris a preferred geometry for titanium tube 16.

Certain applications for bulk material irradiation systems may requirethe pressure in the interior of tube 16 to be quite high. For example,it may be desired for the irradiation system to feed directly into aproduct packaging system that presents a significant amount of backpressure to the system. Various types of pumps are known in the art thatare capable of maintaining pressures of 300 pounds per square inch (psi)or more. This pressure is sufficiently high that tube 16 must bedesigned to sustain such pressure continuously or a serious systemfailure might occur.

FIG. 23 is a diagram of tube 16 having a structure that is a combinationof the structures of FIGS. 21 and 22. Wall 260 of the internal tube isconstructed of solid titanium that is partially compensated by formingan increased wall thickness at the outer edges of the tube and isgenerally more structurally rigid than the thin walled tube of FIG. 22.The outer tube that contains cooling water 262 is generally ellipticalin shape and surrounds the inner tube with a greater thickness of water262 in the outer edges to efficiently remove heat and to provide theexposure compensation needed to maintain the equivalent absorptioncharacteristic of constant thickness material. Water 262 containedbetween the outer cooling and compensation tube and the inner exposureand material containment tube may be pressurized by the use of a highpressure water pump and a pressure regulator/flow control valve. Theinner material containment tube may have sufficient structural strengthand integrity to maintain some moderate amount of pressure by itself,and with the pressure applied on the exterior surface of wall 260, thepressure contained within is the sum of the water pressure and thepressure containment capacity of the inner tube. The net effect is thatthe pressure that may be sustained by the inner exposure and materialcontainment tube may be increased substantially while still providingthe necessary cooling and exposure compensation.

Maintenance of the spacing between the two tube structures maybeaccomplished by the use of small spacer buttons (not shown) constructedof titanium or some other relatively low radiation absorption materials.These spacers maybe small rod or pin-like structures welded into placeto provide some structural strength enhancement, or they may be simplypressed or wedged into place to insure that they remain in the properlocation. Alternately, the spacing structures may be welded to theinterior tube to hold them in place before the outer tube is slippedover the inner tube assembly. Flange structures may be welded to eachend of the interior and exterior tubes with water ports to provide thepath for the cooling water to enter and exit.

The present invention is a bulk material irradiation system havingmultiple features for effectively and efficiently providing consistentand controllable irradiation dosage to a flowable bulk material such asground beef. For example, a material handling system, double-sidedirradiation exposure system, linear electron beam locating system,exposure module system, dosimetry carrier system, and exposurecompensation system are disclosed in conjunction with the bulk materialirradiation system of the invention. The many aspects of the presentinvention improve the ability to effectively irradiate bulk materials,which enables the expansion of product types that may desirably beirradiated for increased safety.

Although the present invention has been described with reference topreferred embodiments, workers skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention.

What is claimed is:
 1. A bulk material irradiation system comprising: aninput for inserting bulk material; a bulk material tube connected to theinput and forming a path for bulk material flow; a pressurizing assemblyconnected to the bulk material tube for forcing the bulk material toflow through the bulk material tube, the pressurizing assemblycomprising a pump assembly operable to remove oxygen from the bulkmaterial flowing through the bulk material tube; an irradiation assemblyproviding ionizing radiation that penetrates a full thickness of thebulk material to irradiate the bulk material passing adjacent to theirradiation assembly in the bulk material tube; and an output forirradiated bulk material to exit the bulk material tube.
 2. The bulkmaterial irradiation system of claim 1, wherein the irradiation assemblyis operable to provide ionizing radiation to irradiate the bulk materialpassing adjacent to the irradiation assembly in the bulk material tubefrom two opposite sides.
 3. The bulk material irradiation system ofclaim 2, wherein the irradiation assembly comprises: an electronaccelerator producing an electron beam in a path defining an axis; amagnet assembly for deflecting and scanning the electron beam across thebulk material in the bulk material tube from a first side and from asecond opposite side; a first scan horn offset from the axis defined bythe electron beam, the first scan horn providing a medium forpropagation of the electron beam from the magnet assembly to the bulkmaterial tube on the first side of the bulk material in the bulkmaterial tube; and a second scan horn offset from the axis defined bythe electron beam, the second scan horn providing a medium forpropagation of the electron beam from the magnet assembly to the bulkmaterial tube on the second side of the bulk material in the bulkmaterial tube.
 4. The bulk material irradiation system of claim 3,further comprising: a control system for operating the magnet assemblyto evenly space successive scan steps of the electron beam on the bulkmaterial in the bulk material tube.
 5. The bulk material irradiationsystem of claim 1, wherein the irradiation assembly comprises: anelectron accelerator producing an electron beam, the electron beamcomprising a plurality of successive electron pulses; and a powercontroller for selectively adjusting a power level of each of thesuccessive electron pulses of the electron beam produced by the electronaccelerator.
 6. The bulk material irradiation system of claim 1, whereinthe irradiation assembly includes a sensor for measuring a level ofionizing radiation delivered to the bulk material.
 7. The bulk materialirradiation system of claim 6, wherein the irradiation assemblycomprises: an electron accelerator producing an electron beam, theelectron beam comprising a plurality of successive electron pulses; anda power controller for selectively adjusting a power level of each ofthe successive electron pulses of the electron beam produced by theelectron accelerator based on the level of ionizing radiation measuredby the sensor to control a dosage of ionizing radiation delivered to thebulk material.
 8. The bulk material irradiation system of claim 1,wherein the irradiation assembly comprises: a source of ionizingradiation; a conduit for providing a radiation propagation path betweenthe source of ionizing radiation and the bulk material tube; a foilbetween the conduit and the bulk material in the bulk material tube; anda gas flow path adjacent to the foil opposite the bulk material tube forreceiving a flow of gas pressurized to a level approximately equal to alevel of pressurization in the bulk material tube.
 9. The bulk materialirradiation system of claim 8, further comprising: a sensor formeasuring the level of pressurization in the bulk material tube; and agas flow adjustment mechanism for adjusting the pressurization of thegas flow in the gas flow path to maintain the level of pressurization othe gas flow path approximately equal to the level of pressurization inthe bulk material tube.
 10. The bulk material irradiation system ofclaim 8, further comprising: a carrier frame rigidly attached to thebulk material tube, the foil being bonded to the carrier frame.
 11. Thebulk material irradiation system of claim 1, wherein the at least onebulk material tube comprises a plurality of bulk material tubes eachoffset from adjacent tubes in an alternating pattern.
 12. The bulkmaterial irradiation system of claim 1, wherein the bulk material tubeis composed of titanium and a laminate layer for contacting bulkmaterial on an inner surface of the bulk material tube.
 13. The bulkmaterial irradiation system of claim 12, wherein the laminate layer iscomposed of stainless steel.
 14. The bulk material irradiation system ofclaim 1, further comprising: a dosimetry carrier entry port in the bulkmaterial tube upstream from the irradiation module; and a dosimetrycarrier exit port in the bulk material tube downstream from theirradiation module.
 15. The bulk material irradiation system of claim14, wherein the dosimetry carrier entry port and the dosimetry carrierexit port are configured to receive a dosimetry carrier having a shapeconforming to an inner circumference of the bulk material tube.
 16. Thebulk material irradiation system of claim 14, wherein the dosimetrycarrier exit port is movable between a first position for allowing bulkmaterial to pass through the bulk material tube and a second positionfor diverting a flow of bulk material away from the bulk material tubeinto a waste area.
 17. The bulk material irradiation system of claim 1,wherein the bulk material tube is elliptical in shape and has a wallwith a thickness that is thicker around edge portions of the bulkmaterial tube than around a central portion of the bulk material tube.18. The bulk material irradiation system of claim 1, further comprisinga liquid around the wall of the bulk material tube.
 19. The bulkmaterial irradiation system of claim 1, wherein the bulk material tubehas an outer wall that is rectangular in shape and an inner wall that iselliptical in shape, a region between the outer wall and the inner wallincluding a liquid with an irradiation absorption characteristic thatapproximately matches an irradiation absorption characteristic of thebulk material in the bulk material tube.
 20. The bulk materialirradiation system of claim 1, wherein the bulk material is ground beef.21. The bulk material irradiation system of claim 1, wherein the bulkmaterial fills substantially all of a volume of the bulk material tube.22. The bulk material irradiation system of claim 1, wherein the bulkmaterial tube is configured so that there is no straight line path forradiation to exit the irradiation assembly through the input or theoutput.